The invention relates generally to a phased array coil assembly and more specifically to a phased array coil assembly for use in Magnetic Resonance Imaging (MRI).
Magnetic resonance imaging systems have found increasing applicability for a variety of imaging tasks, particularly in the medical field. Such systems also typically include coil assemblies for generating radio frequency (rf) magnetic fields used to control and excite spin systems in a subject of interest, such as in soft tissues of a patient. A body coil is typically employed for generating a highly uniform rf magnetic field transverse to the direction of the main, magnetic field direction. A series of gradient coils generate spatially varying magnetic fields to select a portion of the subject to be imaged, and to spatially encode sensed signals emitted by unitary volumes within the selected slice. The field gradients may be manipulated to orient the selected image slice, and to perform other useful imaging functions. Signals of a particular frequency acquired during application of the field gradient may be assumed to originate at a given position within the field gradient. The application of such a field gradient is also referred to as frequency encoding.
Sensing coils are employed in conventional MRI systems and are adapted to the particular type of image to be acquired. Such sensing coils are highly sensitive to emissions from the subject positioned within the primary and gradient fields. Such emissions, collected during data acquisition phases of imaging, serve to generate raw data signals which may be processed to extract information relating to the nature and location of different tissue types in the subject. Where the region to be imaged is relatively small, a single channel surface coil may be employed. For example, a linearly polarized shoulder coil is typically employed for producing images of a human shoulder. For larger images, large single coils may be employed, or multiple coils may be used, such as in “phased array” arrangements. However, the use of large surface coils tends to result in lower signal-to-noise ratios in the acquired image data. Generally, surface coils have limited field of view (FOV) and lead to inhomogenous spatial uniformity. Phased array coils overcome this problem. Phased array coil assemblies are, therefore, commonly employed to produce images of larger areas, while providing an acceptable signal-to-noise ratio. Typically, phased array coil assemblies consist of multiple, non interacting coils, having similar SNR as a surface coil, but the combined FOV of a larger coil. In addition, the penetration of array coils compensates for limited penetration of individual coils.
In a typical phased array arrangement, several adjacent coils are provided for receiving the signals emitted by the spin systems of interest during the signal acquisition phase of imaging. The output signals from each of several adjacent coils are independently amplified in the preamplifiers prior to processing of the signals for generation of the image data.
Use of phased array coils also impacts the formation of magnetic resonance (MR) images (either two dimensional i.e 2D or three dimensional i.e 3D) which takes place in the complex Fourier domain, called k-space. In a typical MR system, as described above, gradients of varied strength are applied in a perpendicular direction to the frequency encoding gradient using gradient coils, prior to acquisition of the signal, to thereby twist the phase of the nuclear spins by varied amounts. The application of such additional gradients is referred to as phase encoding. Frequency-encoded data sensed by the detector coils after a phase encoding step is stored as a line of data in the k-space matrix. Multiple phase encoding steps are performed in order to fill the multiple lines of the k-space matrix. An image may be generated from this matrix by performing a two-dimensional or three dimensional Fourier transformation of the matrix to convert this frequency information to spatial information representing the distribution of nuclear spins or density of nuclei of the image material. One of the time limiting factor in the formation of MR images is the process of filing up k-space with data, which is done in successive manner, one line at a time.
To overcome these inherent limits, several techniques have been developed to effectively simultaneously acquire multiple lines of data for each application of a magnetic field gradient. These techniques, which may collectively be characterized as “parallel imaging techniques”, use spatial information from the radio frequency (rf) detector coils to substitute for the encoding which would otherwise have to be obtained in a sequential fashion using field gradients alone. The use of multiple rf detector coils has been shown to decrease image acquisition time. Several coil geometries have been attempted for use with these parallel imaging techniques for enhancing image acquisition time. Typically these include linear arrays of coils and spatial information is acquired from minimally overlapping coils. Multiplexed array having a circular symmetry having three component coils have also been suggested. These arrangements have limitations in terms of using complex computations or implementation limitations when larger number of coils are desired.
It is therefore desirable to have coil geometries which significantly reduce the computation load and lead to easy implementation in MRI systems.